1. Field of the Invention
The present invention relates to a system for an optical recording medium, and more particularly, to a servo method for a push-pull type optical recording medium.
2. Background of the Related Art
With the growth of audio and video media, an optical recording/reproducing apparatus for recording and reproducing video and audio data in a semi-permanent optical recording medium has been developed.
An optical recording medium, and especially an optical disc used for data recording and reproduction in the optical recording/reproducing apparatus is classified into three types according to its function and purpose: a read only memory (ROM) type, write one read many (WORM) type on which data can be written only once, and rewritable type on which data can be repeatedly written.
Here, as the ROM type optical disc, there exist a compact disc read only memory (CD-ROM), digital versatile disc read only memory (DVD-ROM), etc., and as the WORM type optical recording medium, there exist a recordable compact disc (CD-R) on which data can be written only once, recordable digital versatile disc (DVD-R) on which data can be written only once, etc.
Meanwhile, as the rewritable optical disc, there exist a rewritable compact disc (CD-RW), rewritable digital versatile disc (DVD-RW, DVD-RAM, and DVD+RW), etc.
Various methods of controlling a tracking such as a three-beam method, push-pull (PP) method, differential phase detection (DPD) method, etc., have been used in the optical discs of a CD or DVD series.
Here, according to the three-beam method, a pair of side beam spots for the tracking are arranged in front and rear of a main beam spot for decoding information, respectively. That is, the side beams are arranged opposite to each other from a track center by ¼ of a track width. This arrangement of the side beams is for preventing an omission of detected signals and for avoiding a cross talk from an adjacent track.
At this time, an optical detector includes a center optical detection element for detecting a light quantity of the main beam, and side optical detection elements for detecting a light quantity of the side beam. For example, as shown in FIG. 1, the center optical detection element is composed of four optical detection elements PDA, PDB, PDC, and PDD divided in a signal track direction and in a radial direction of the optical disc, respectively. Two side optical detection elements PDE and PDF are positioned on upper and lower ends of the center optical detection element, respectively. Here, a tracking error signal is obtained by calculating “e−f” with respect to electric signals e and f outputted from the side optical detection elements PDE and PDF.
Also, according to the push-pull (PP) method, the optical detection elements of the optical detector are divided into two parts in the track direction, and a tracking error signal is detected from a light quantity balance of the two-part optical detection elements. Specifically, this method uses the fact that the intensity distribution of light, that is diffracted and reflected by a pit and then incident again to an object lens, varies according to the relative variation of positions of the pit and spot.
At this time, if the shadow of the pit is equally detected by the both optical detection elements, the tracking error (TE) signal becomes “0”, and this state is called a tracking-on (or on-track) state. On the contrary, if the optical beam deviates left and right from the track center, the tracking error (TE) signal has a positive (+) or negative (−) value, and this state is called a tracking-off (or off-track) state.
In case that the optical detector is divided into 4 optical detection elements PDA, PDB, PDC, and PDD in the signal track direction and radial direction of the optical disc, the tracking error signal according to the PP method can be obtained by calculating “(a+d)−(b+c)” with respect to the electric signals a, b, c, and d outputted from the optical detection elements of the optical detector. At this time, in case that the optical detector is divided into two parts in the track direction, the tracking error signal (=I1−I2) is detected from the light quantity balance of two photodiodes I1 and I2. That is, “a+d” of FIG. 1 corresponds to I1, and “b+d” corresponds to I2.
If it is assumed that the optical detector is the same as in FIG. 1, the main beam push-pull (BPP) signal can be obtained by calculating “(a+d)−(b+c)” with respect to the electric signals a, b, c, and d outputted from the center optical detection element of the optical detector, and the side beam push-pull (SPP) signal can be obtained by calculating “e−f” with respect to the electric signals e and f. Also, the differential push-pull (DPP) signal can be obtained by calculating “MPP-SPP”. Here, in case that the optical detector is composed of 4 divided optical detection elements PDA, PDB, PDC, and PDD in the signal track direction and radial direction of the optical disc, or 2 divided optical detection elements I1 and I2 in the track direction, the PP signal and MPP signal have the same concept.
At this time, the MPP method has several conditions. According to one among them, if the wavelength of the light is λ and the depth of the pit is λ/4, i.e., if the diffraction by the pit is most effective and the depth of modulation becomes maximum, the tracking error signal cannot be obtained through the MPP method. In other words, since the pattern becomes symmetric when the depth of the pit is λ/4, the tracking error signal cannot be obtained through the 2-divided optical detector.
Meanwhile, the DPD method is an improvement of the MPP method. In the same manner as the MPP method, the DPD method uses the intensity distribution of light according to the relative positional change of the beam and pit, but it uses the 4-divided optical detector instead of the 2-divided optical detector.
Specifically, according to the DPD method, the intensity distribution of the light is received through the 4-divided optical detection elements, and the tracking error signal is generated through the detection of the phase difference in the radial direction.
Accordingly, the tracking error signal is outputted even if the depth of the pit is λ/4, and is not much affected by movement of the beam on the optical detector as well.
In case that the optical detector is divided into 4 optical detection elements PDA, PDB, PDC, and PDD in the signal track direction and radial direction of the optical direction, the DPD method obtains the tracking error signal through the detection of the phase difference between the electric signals of “a+c” and “b+d” based on the RF signal of “a+b+c+d” obtained from the electric signals a, b, c, and d outputted from the respective optical detection elements.
For example, the existing CD generates the tracking error signal using the three-beam method, and the CD recorder generates the tracking error signal using the differential push-pull (DPP) method. At this time, in case that the signal outputted from the center optical detection element is received and the tracking error signal is detected using the MPP method, the quantity of the MPP signal becomes great, and a DC offset is produced through a lens shift.
Accordingly, the CD recorder generates the tracking error signal in a manner that the DC offset is minimized using the DPP (DPP=MPP−SPP) method, i.e., using the signal obtained in the main push-pull (MPP) method and side push-pull (SPP) method, to solve the DC offset problem in the MPP method.
Also, the DVD-ROM generates the tracking error signal using the DPD method. Specifically, since the depth of the pit is λ/4 in case of the DVD-ROM, the tracking error signal cannot be detected through the MPP method. Thus, the DVD-ROM obtains the tracking error signal using the DPD method. Also, the DVD-R/RW detects the tracking error signal using the DPD method in case of reproducing a region where the signal is recorded, while it detects the tracking error signal using the MPP method in case of recording the signal. Also, the DVD-RAM detects the tracking error signal using the DPD method only with respect to a pre-pit region, while it detects the tracking error signal using the MPP method with respect to other regions.
Meanwhile, after the tracking error signal is detected using one among the above-described methods, a tracking servo should be performed using the detected tracking error signal. At this time, as the tracking servo driving method, there exist a method of driving the object lens only, and a method of driving the whole optical pickup. Here, in order to move the whole optical pickup, a sled motor should be driven by a sled servo.
Also, in case of a seek or search operation for searching a desired position on the optical disc, there exist a method of driving the object lens only, and a method of driving the whole optical pickup. Specifically, in case of a rough seek for Specifically, in case of a rough seek for jumping over several hundred tracks to several thousand tracks to reach a target track, the whole optical pickup is moved near the desired track through the drive of the sled motor, and then in case of a fine seek for jumping over several hundred tracks or less, the target track is sought only using the object lens of a tracking actuator.
Meanwhile, in case that the tracking error signal is detected by the MPP method as the object lens of the tracking actuator is moved left and right through the lens shifting method during the tracking servo, the spot is moved on the optical detector, and the DC offset is produced in the tracking error signal. That is, the DC offset is produced in the tracking error signal through the lens shift although the optical beam is positioned on the center track, and this causes the center of the reflected light not to coincide with the center of the optical detector. Here, The DC offset means that the position of the spot leans to the upper or lower part of the optical detector. In this case, since the optical beam is on the track center, the DC offset state should be maintained as it is.
As shown in FIG. 2A, if it is assumed that B indicates the normal focusing state that no lens shift is performed and A and C indicate the shifted positions of the object lens, FIGS. 2B to 2D show examples of the reflected light detected by the optical detector. That is, if it is assumed that the optical beam is positioned on the track center, the center of the reflected light coincides with the center of the optical detector as shown in FIG. 2C in case that the lens shift does not exist, and the center of the reflected light does not coincide with the center of the optical detector due to the DC offset as shown in FIGS. 2B and 2D in case that the lens shift is performed. Here, FIG. 2B shows an example that the DC offset is produced toward I2, and FIG. 2D shows an example that the DC offset is produced toward I1.
Then, a servo control section in the conventional optical recording/reproducing apparatus as shown in FIG. 3 judges that the optical beam deviates from the track center, and produces a control signal for controlling the center of the reflected light and the center of the optical detector to coincide with each other. Accordingly, the optical beam may deviate from the track center.
FIG. 3 is a block diagram illustrating the construction of the general optical disc recording/reproducing apparatus. Referring to FIG. 3, an optical pickup 102, under the control of a servo control section 104, puts the optical beam condensed by the object lens on the signal track of the optical disc 101. The light reflected from a signal recording surface is condensed again by the object lens, and then incident to the optical detector for detecting a focus error signal and tracking error signal.
For example, the optical detector is composed of a plurality of optical detection elements as shown in FIG. 1, and electric signals in proportion to the light quantities obtained by the respective optical detection elements are outputted to an RF and servo error generating section 103.
The RF and servo error generating section 103 detects an RF signal for data reproduction, and the focus error (FE) signal, tracking error (TE) signal, etc., for servo control from the electric signals outputted from the respective optical detection elements of the optical detector.
The detected RF signal is outputted to a data decoder (not illustrated) for reproduction, and the servo error signals such as the FE signal, TE signal, etc., are outputted to the servo control section 104.
The servo control section 104 outputs a drive signal for controlling the focus to a focus servo driving section 105 by processing the focus error (FE) signal, and outputs a drive signal for controlling the tracking to a tracking servo driving section 106 by processing the tracking error (TE) signal.
Then, the focus servo driving section 105 moves the optical pickup 102 up and down by driving the focus actuator in the optical pickup 102, so that the optical pickup 102 follows the up/down movement of the rotating optical disc 101.
The tracking servo driving section 106 moves the object lens of the optical pickup 102 in the radial direction by driving the tracking actuator in the optical pickup 102, and thus corrects the position of the beam to follow a specified track.
Also, in case of moving the whole optical pickup, a sled servo driving section 107 receives a sled control signal and forward (or reverse) signal from the servo control section 104, and drives a sled motor 108 to directly move the optical pickup body in a desired direction. Specifically, the servo control section 104 generates the sled control signal using the tracking error signal, and outputs the sled control signal to the sled servo driving section 107. That is, the tracking error signal becomes symmetric, i.e., positive (+) or negative (−), when the optical pickup 102 is not positioned on the center of the track. Meanwhile, the sled servo driving section 107 drives the sled motor 108 so that the rotation of the sled motor 108 corresponds to the quantity of output of the optical pickup 102, and this means that the control voltage of the sled motor 108 changes the rotating number of the sled motor to change the speed of the sled motor. For example, the forward signal inputted to the sled servo driving section 107 is the positive or negative voltage applied to the sled motor 108, and makes the sled motor 108 rotate to move the optical pickup 102 in the desired direction.
For example, as shown in FIG. 4, if it is assumed that a control reference voltage (i.e., center voltage TEref) is determined and the tracking error (TE) signal is inputted from the RF and servo error generating section 103, the servo control section 104 generates the tracking control signal so that the difference between the tracking error (TE) signal and reference voltage Teref (Δε=TE−TEref) becomes “0” if Δε is not “0”. Then, the tracking servo driving section 106 generates a tracking drive (TD) signal in proportion to the tracking control signal outputted from the servo control section 104, and drives the tracking actuator in the optical pickup 102. If the tracking actuator is driven, the tracking drive voltage increases to move the object lens of the optical pickup 102 in the radial direction.
At this time, the servo control section 104 considers that the optical beam follows the track center when Δε becomes “0”.
However, since the DC offset exists in the tracking error (TE) signal that has already inputted through the lens shift, the tracking drive (TD) signal outputted from the tracking servo driving section 106 has the DC offset as much as α. Accordingly, the tracking servo driving section 106 is overdriven as much as α.
As a result, the optical beam deviates from the track center. In this case, the tracking actuator cannot follow the track center often. Especially, if the DC offset becomes great, the tracking servo cannot follow the corresponding track, and thus the tracking servo becomes off. This causes the recording/reproduction of data to be difficult.
In particular, since the DVD recorder controls the tracking only using the MPP method during the recording operation, it is greatly affected by the DC offset generated by the lens mechanism. That is, as high-density DVDs are provided, the DC offset exerts a great effect on the cross talk or cross eraser (i.e., erasing data recorded on the neighboring track by affecting the neighboring track during the recording operation) Also, in the DVD-R/RW having a small PP signal, the gain of the sled motor should be increased to obtain the track control, and this causes a fatal influence to be exerted by the DC offset.
Meanwhile, even in case of moving the whole optical pickup by driving the sled motor 108, the DC offset may cause a problem.
Specifically, a dead zone exists in the sled motor 108. The dead zone means a region where the sled motor 108 cannot move. The sled motor 108 does not move until the input voltage becomes a predetermined level due to an initial frictional force.
If a voltage less than the predetermined level is applied to the sled motor 108, the sled motor 108 does not operate even with a power consumption, and the object lens is continuously moved in the radial direction.
Accordingly, in order to quickly drive the sled motor 108, i.e., in order to make the sled motor 108 quickly escape from the dead zone, the conventional method uses the increase of the gain of the sled motor 108. However, the increase of the gain may cause a transient response.